In aircraft construction, a stringer or longeron or stiffener is a thin strip made of carbon fiber or aluminum or other metallic alloy. Some strips are co-bonded on a cured skin in order to build a part of the aircraft (e.g. wings, HTP, etc).
A very common type of stringers is the one with a “T” shaped cross section defining a stringer web and a stringer foot.
The most common method of manufacturing a composite “T” shaped stringer for an aircraft comprises a first step of hot-forming the laminates in order to achieve semi-stringers geometry with an “L” shaped cross section. A second step is placing two hot-formed semi-stringers together forming a “T” shaped stringer. This “T” shaped stringer is co-bonded on a cured skin with an adhesive line between the stringer and the skin. A third step is a curing cycle, in which curing tools are placed following the contour of the hot-formed “T” shaped stringer inside a vacuum bag during the curing cycle. An invar angle is used as a curing tool in those cases.
The vacuum bag consists of a release film, an airweave and a vacuum bag film. The material of the vacuum bag film is a very thin film with a great elongation capacity to adapt perfectly to the invar geometry, because any corner near 90° can cause a break in the vacuum bag. The milder the geometry of the invar angles and retainers is, the better the behaviour of the vacuum bag film is.
During said curing cycle, the uncured resin of the stringers and the adhesive flow. Retainers are used to prevent the uncured resin and the adhesive from flowing. From the state of the art it is known that retainers have to be used in the upper part of the invar angle in order to prevent the resin from flowing through this upper part. Usually those retainers are silicone retainers placed into slots manufactured in the upper part of the invar angle.
Retainers are also placed in the edges of the invar angles near to the stringer foot in order to prevent the resin and the adhesive from flowing through those edges. These retainers are usually made of silicone and they are placed in slots also manufactured in the invar angles.
The resin and adhesive have to be prevented from flowing in the beginning and the end of the stringer. Nowadays, the materials used for the retainers placed in the beginning and the end of the stringers are cork, airpad, silicone, adhesive tape . . . .
When retainers of cork or airpad are used, they have to be cut into strips in order to have a “T” section with the same dimensions as the stringer cross-section. These retainers made of cork or airpad are placed directly in contact with the uncured stringer. The invar angles used for curing the stringers are placed following its contour, so the retainers that have the same dimensions as the “T” shaped cross section are laterally covered by the invar angle.
One of the disadvantages of these retainers placed at the beginning and at the end of the stringers is the difficulty to remove them. When working with cork retainers the reason of this disadvantage is that during the curing cycle, the uncured resin of the stringers flows between the strips of cork that form the retainer. The adhesive of the cork strips that form the retainer is mixed with the resin flow. After the curing cycle it is necessary to cut the cork. Furthermore, the “T” shaped section of the beginning and the end of the stringer has not a good surface quality.
When using retainers made of airpad during the curing cycle, the retainer loses part of its geometry. When it happens, the retainer and the resin are mixed and the stringer geometry is deformed.
Another disadvantage of the retainers known from the prior art is that they have to be replaced for each manufactured specimen.
It was therefore desirable to find a retainer that can be used in more than one manufactured specimen, provides a better surface quality, is easy to install and remove and allows a better adaptation of the vacuum bag to the beginning and the end of the stringer.